Phnom Penh , Cambodia -LRB- CNN -RRB- -- The cause of a mysterious illness that has claimed the lives of more than 60 Cambodian children has been determined , medical doctors familiar with the investigation told CNN on Wednesday .

A combination of pathogens , disease-causing micro-organisms , is to blame for the illness , the World Health Organization , in conjunction with the Cambodian Ministry of Health , has concluded , the doctors said .

The pathogens include enterovirus 71 , which is known to cause neurological disease ; streptococcus suis , which can cause infections like bacterial meningitis in people who have close contact with pigs or with pork products ; and dengue , which is transmitted by mosquitoes .

The inappropriate use of steroids , which can suppress the immune system , worsened the illness in a majority of the patients , the doctors said . The World Health Organization -LRB- WHO -RRB- is expected to advise health care workers to refrain from using steroids in patients with signs and symptoms of the infection , which include severe fever , encephalitis and breathing difficulties .

While not all the microorganisms were present in each patient , doctors concluded the illness was caused by a combination of them and worsened by steroid use .

The WHO sources did not want to be identified because the results of the health organization 's investigation have not yet been made public .

`` I 'm very confident for the reason of the epidemic , '' said Dr. Phillipe Buchy , chief of virology at the Institut Pasteur in Cambodia and one of the doctors who cracked the case .

`` The first thing that goes through your mind is , is this one of the usual suspects you have n't detected before ? '' said Dr. Arnaud Tarantola , chief of epidemiology and public health at the Institut Pasteur . `` If it is , has it mutated , or changed in a way that it causes more severe disease ? Or is it something completely new ? ''

On the steroids issue , Tarantola said , `` When you have a dying child , you try to use what you have at hand , and they were right to try that . '' But , he acknowledged , `` from the cases we reviewed , almost all of the children died , and almost all of them had steroids . ''

Parents face anxious wait over mystery illness

`` I think we can close the case and move ahead asking different questions , '' Buchy said . `` Not what is the illness , but now , how long has the virus been circulating ? What is the extent of the circulation of the virus ? How many mild diseases are we missing ? That 's the next step . ''

Over the past four months , doctors at Kantha Bopha Children 's Hospitals in Phnom Penh have been faced with the mysterious syndrome , which kills children so fast that nearly all of those infected with it die within a day or two of being admitted to the hospital .

Dr. Beat Richner , head of the children 's hospitals -- which cared for 66 patients affected by the illness , 64 of whom died -- said that no new cases of the illness had been confirmed since Saturday .

Other hospitals in the country have reported similar cases , but far fewer than the children 's hospitals in the capital , which are the most popular .

In the last hours of their life , the children experienced a `` total destruction of the alveola -LRB- e -RRB- in the lungs , '' Richner said . Alveolae are the air sacs where oxygen enters the bloodstream .

Most of the children who have contracted the illness have come from the south of the country , though health officials can not find what is known as a cluster -- a lot of cases coming from one specific area .

By June 29 , the WHO had been contacted and Cambodian officials were scrambling to instruct health providers across the country to spread information about the illness as quickly as possible .

Officials search for clues in disease killing Cambodia 's children

The WHO and the Cambodian authorities ' announcement of the situation drew criticism from Richner , who said they were `` causing unnecessary panic . ''

The WHO said the unexplained nature of the outbreak obliged it to communicate the information .

Over the weekend , lab tests linked enterovirus 71 -LRB- EV71 -RRB- to some of the cases . But the tests did n't solve the whole puzzle and health officials continued their investigations , noting the detection of other elements like streptococcus suis and dengue .

The link to EV71 does not particularly help in the treatment of the illness , as there is no effective antiviral treatment for severe EV71 infections and no vaccine is available .

In milder cases , EV71 can cause coldlike symptoms , diarrhea and sores on the hands , feet and mouth , according to the journal Genetic Vaccines and Therapy . But more severe cases can cause fluid to accumulate on the brain , resulting in polio-like paralysis and death .

Outbreaks of the enterovirus `` occur periodically in the Asia-Pacific region , '' according to the CDC . Brunei had its first major outbreak in 2006 . China had an outbreak in 2008 .

Adults ' well-developed immune systems usually can fend off the virus , but children are vulnerable to it , according to the CDC .

`` It looks like -LRB- EV71 -RRB- has emerged strongly , probably because it had n't circulated with the same intensity in the past years , '' Tarantola said .

Reported cases of streptococcus suis have risen significantly in recent years , notably in Southeast Asia , according to a paper that appeared last year in Emerging Infectious Diseases , a journal published by the Centers for Disease Control and Prevention in Atlanta .

The rainy season in Cambodia , which lasts from May to October , is a key problem in trying to control diseases like dengue . Because of a lack of indoor plumbing in many homes , people collect rainwater in vats , creating potential breeding grounds for mosquitoes .

In Cambodia , as with many places around the world , parents first try treating their child at home . If that does n't work , they typically then go to a local clinic . A hospital visit , which often involves a long trip , is a last resort .

Mystery illness claiming dozens of lives

CNN 's Sara Sidner and Tim Schwarz contributed to this report .

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Doctors say the inappropriate use of steroids worsened many cases

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No new cases of the condition have been confirmed since last Saturday

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The pathogens include enterovirus 71 , streptococcus suis and dengue

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Most of the patients have come from southern Cambodia